1. Field of the Invention
The present invention relates to a liquid crystal driving circuit.
2. Description of the Related Art
In a segment-display type or a simple matrix driving type liquid crystal panel, a common signal and a segment signal are supplied to a common electrode and a segment electrode, respectively, and on/off controlled in accordance with a voltage (potential difference) between two electrodes, in general.
In these liquid crystal panels, performing time-division driving enables display of more segments (pixels) than the number of output terminals of an IC for driving a liquid crystal. For example, in a liquid crystal panel with m numbers of common electrodes and n numbers of segment electrodes, performing 1/m duty cycle driving enables displaying m×n segments at maximum. Further, in time-division driving, 1/S bias driving is performed so that each signal can obtain (S+1) potentials. For example, in FIG. 4 of Japanese Patent Laid-Open Publication No. H10-10491, disclosed is an LCD driving power circuit used for ⅓ bias driving.
Here, a configuration of a common liquid crystal driving circuit that performs time-division driving and an example of an operation thereof are illustrated in FIGS. 10 and 11, respectively.
As illustrated in FIG. 10, intermediate potentials V1 and V2 obtained by dividing a power supply voltage V0 (=VDD−VSS) by resistors R1 to R3 are supplied, in addition to power supply potentials VDD and VSS on a high-potential side and a low-potential side, to a common-signal output circuit 7 and a segment-signal output circuit 8. Therefore, in this liquid crystal driving circuit, ⅓ bias driving (S=3) is performed.
Further, FIG. 11 illustrates an operation of the liquid crystal driving circuit that performs ¼ duty cycle driving (m=4). As illustrated in FIG. 11, the potential of a common signal COMi (1≦i≦m), during a single period T1, is at the power supply potential VDD or VSS for a ¼ period and at the intermediate potential V1 or V2 for a ¾ period. On the other hand, segment signals SEGj and SEGj′ (1≦j, j′≦n) are at potentials according to turning on or off of four segments corresponding to segment electrodes to which the signals are supplied.
As described above, use of the 1/m duty cycle and 1/S bias driving method enables displaying more segments than the number of output terminals of the IC for driving a liquid crystal.
The common electrode to which the common signal COMi is supplied and the segment electrode to which the segment signal SEGj is supplied are capacitively-coupled through liquid crystal, and thus, beard-like spike noise might be generated in one of the signals, which is caused by change in potential of the other of the signals. Thus, in the liquid crystal driving circuit illustrated in FIG. 10, similar to FIG. 4 in Japanese Patent Laid-Open Publication No. H10-10491, capacitors C1 and C2 are used as stabilizing capacitors so as to absorb the spike noise and to stabilize the intermediate potentials V1 and V2. As illustrated in FIG. 12, such a liquid crystal driving circuit is known that stabilizes the intermediate potentials V1 and V2 using voltage follower circuits configured by operational amplifiers OP1 and OP2, respectively.
However, since the capacitance of the capacitor used as the stabilizing capacitor is large in accordance with the liquid crystal panel, the capacitor is usually provided as an external component, which increases the mounting area of a circuit board. On the other hand, since the output impedance of the operational amplifier which makes up the voltage follower circuit is small, current consumption increases.
Further, if the output impedance of the operational amplifier is not sufficiently small, as illustrated in FIGS. 13 and 14, spike noise Sp is not sufficiently absorbed, which might cause a defective display such as an image remaining in the liquid crystal panel. Here, as an example, FIG. 13 illustrates spike noise Sp generated when the potential of the common signal COM1 is switched while the potential of the segment signal SEGj is at the intermediate potential. Whereas, FIG. 14 illustrates spike noise Sp generated when the potential of the segment signal SEGj′ is switched while the potential of the common signal COM1 is at the intermediate potential.
Thus, in order to ensure favorable display quality, the current consumption of the liquid crystal driving circuit and the mounting area of the circuit board are in a trade-off relationship.